Various nuts and bolts are used as fastener assemblies in manufacturing industry, for example, in modern aircraft construction. For instance, when parts of an aircraft need to be joined during operation and to be disassembled or replaced frequently during maintenance, a threaded fastener may be used to allow both riveting and dismantling functions. In addition, when joints require great strength and rigidity, a threaded fastener may be a better choice than a rivet.
In connection with the utilization of nuts and bolts, washers may also be used to provide an enhanced securement for fastener assemblies. In particular, a locking fastener assembly may be employed to prevent loosening of a threaded fastener element during operation. There are multiple scenarios in which a locking fastener assembly is desirable and required to prevent a nut from unfastening. One of such applications is at the joint of workpieces in close proximity to an airfoil along the flowpath in a gas turbine engine due to aerodynamic considerations which favor tertiary locking for fastener assemblies using wedge lock washers. These wedge lock washers are designed to secure fasteners that have a tendency to rotate or lose friction under the impact of vibration or dynamic loads.
A wedge locking threaded fastener assembly is known in the art. For example, such a locking threaded fastener assembly may include two identical and superposing washer pieces, which are supported on one another in a point-symmetric way and which are arranged with cammed faces in mutual engagement. The two washers are located between the head of the threaded bolt and the flange which separates the washers from the surface of the joint. On one hand, the inwardly facing, cammed faces of the washers include a tooth arrangement such as, for example, wedge shaped teeth. Such a tooth arrangement provides form-fit between the two washer pieces when the screw is being tightened. On the other hand, the outwardly facing surfaces of the washers include serrations which increase friction between the washer and the surfaces of the bolt head and the flange. Such serrations dig into the bolt head and the flange surface in depths wherein they will not be disengaged as the bolt elongates. The purpose of this design is to have the serration teeth “bite” into the associated surfaces to prevent slippage between the washer and the flange and between the washer and the bolt. Motion at the outside faces of the washer pieces would require shearing of the flange and/or bolt head material. The combined results have shown that the wedge locking washers have been very effective against loosening of the nut.
One disadvantage of such serration structure is that the surfaces on the flange and bolt have to be relatively soft in order for the serration teeth to exert the desired biting action. As a result, there are mated impressions on the flange surface caused by the teeth bites. Such impressions create stress concentrations whose effects are difficult to quantify or analyze. Since stresses may lead to flange failure and the ensuing loosening of the fastener and previously joined workpieces, it is desirable to minimize or eliminate flange or coating damages caused by the serrations of the washer pieces of a fastener assembly.
To better answer the challenges raised by the gas turbine industry to produce reliable and high-performance gas turbines, it is therefore desirable to find wedge locking washers which would provide the required securement for the a fastener assembly. Further, reusable wedge locking washers without causing flange damage are desirable as well.